Classic methods of treating neoplastic growth, by surgery, chemotherapy, and/or radiation involve a trade-off between destroying the target tumor and not killing normal cells. A tumor killing modality which does not kill normal cells is obviously desirable because the treatment could be applied as aggressively as needed to kill the tumor without the need to moderate the treatment to avoid killing non-tumor cells.
Boron neutron capture therapy, BNCT, was developed as a tumor treatment modality which is non-lethal to non-tumor cells. In BNCT, compounds containing the stable isotope boron-10 (.sup.10 B) are introduced into tumor i.e. cancer cells. The part of the body to be treated is then irradiated with neutrons, which results in the formation of high energy state .sup.11 B atoms from .sup.10 B atoms. Neither the neutron beams nor the .sup.10 B are lethal to cells. However, the .sup.11 B formed from the .sup.10 B is in a high energy state and subsequently degenerates to .sup.7 Li and .alpha. particles, which particles give rise to high energy ionizing events which kill the tumor cell. Because these particles have a path length of only about 1 cell diameter, surrounding normal tissue is not adversely affected. Therefore, BNCT basically involves providing both non-radioactive .sup.10 B and nonionizing, low-energy level or thermal neutrons in combination in the tumor cells as described by Barth, R. F., Cancer, Vol. 70, No. 12, Dec. 15, 1992, Pp. 2995-3007, incorporated herein by reference.
The key to successful therapy with BNCT is to obtain a high ratio of boron in proliferating (or non-proliferating) tumor cells compared to boron in normal cells and in blood. Various boron-containing compounds have been used in the prior art to localize boron to tumor cells.
Two compounds, p-carboxybenzeneboronic acid and sodium decahydrodecaborate, have been reported to produce high ratios of boron in tumor cells compared to normal cells. BNCT treatment with these compounds, however, failed due to vascular damage caused by high blood levels of boron.
BNCT therapy with inorganic boron containing compounds, borax (Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O) and sodium pentaborate (Na.sub.2 B.sub.10 O.sub.16.10H.sub.2 O), was found to result in treatment failure because of the lack of localization of boron within tumor cells. A boronated porphyrin compound, 2,4-divinyl-nido-o-carboranyldeuteroporphoryin IX ("VCDP") was found to result in a tumor/normal cell boron ratio of 4:1 in mice. However, this compound is unacceptable for use for BNCT because the mortality of the mice receiving the compound was high, about 10%.
Other compounds that have been tested with varying degrees of success for their ability to localize boron in tumor cells include mercapto compounds such as Na.sub.2 B.sub.12 H.sub.11 SH, thiouracils, purines, pyrimidines, and boronophenylalanine (BPA). BPA has been shown to produce a ratio tumor/normal cell boron ratio of between 3 and 4 to 1 without the deleterious effects seen, for example, with porphyrin.
Other modalities which have been attempted with limited success to localize boron in tumor cells include monoclonal and polyclonal antibodies and encapsulating complexes such as liposomes, microspheres, and low density lipoproteins.
The present invention contributes to a solution to the problem of obtaining a high ratio of tumor boron/normal tissue boron, and thereby effective BNC therapy. Using the novel compounds of the invention, ratios as high as 50:1 or higher may be obtained, without harmful effects to normal tissue.
As stated above, BNCT relies upon the ability to irradiate tumor cells having a high concentration of .sup.10 B with neutrons of suitable energy level. Such neutrons are currently provided by nuclear reactors. Nuclear reactor-derived neutrons are known as described for instance, in Barth, Cancer Vol. 70, No. 12, cited and incorporated herein above. The novel boron-containing compounds are suitable for use with irradiation by neutron beams generated by such conventional nuclear reactors and conceivably could be used with devices of emerging technologies, for example, epithermal neutron beam, californium-252, spallation and low-energy accelerator sources also as described in Barth.
As described above, BNCT is based on generating highly reactive unstable boron-11 from boron-10, the active agent in boron neutron capture therapy. Boron compounds found in nature, which can be used as starting materials for the reactions giving the compounds of the invention, generally are isomeric mixtures which contain both boron-10 and boron-11 (stable form) isotopes in a ratio of approximately 20% boron-10 and 80% boron-11. Accordingly, it is desirable to increase (or enrich) the proportion of boron-10 in the starting boron compounds (over the proportion of stable boron-11) to beyond 20%, preferably above 80%, ideally to 100% to generate a higher proportion of the highly unstable form of the isotope, boron-11. Enriched boron-10 compounds are available commercially such as from Boron Biologicals, Inc., Raleigh, N.C. or from Eagle-Pitcher Industries, Inc., Miami, Okla. Such boron-10 enriched materials are very useful in the practice of the invention.
The amino acid compounds of the invention are D- and L- racemates due to the chiral configuration of the molecule which lend themselves to resolution of the stereoisomers into the L- and D-enantiomers by methods well-known in the art or can be purchased from commercial sources. Of the enantiomers, the L-form is preferrred. Such optically pure form of the compounds of the invention, especially the respective L-forms are very useful for BNC therapy.
Methods for separation of the desired enantiomer from the stereoisomers as by enzymatic resolution are known, as reported for instance, by Coderre et al. in Cancer Research 47, 6377-6383 (1987) Selective Targeting of Boronophenylalanine to Melanoma in BALB/c Mice for Neutron Capture Therapy, which is incorporated herein by reference.
When the L-enantiomer of compound VIII (identified further below) separated from the racemate mixture, is tested in BNC therapy by injection in rats, high cellular uptake is observable followed by death of the cancer cells after irradiation. Comparable results are expected when the L-enantiomers of other compounds of the invention are used in BNCT.